Remove Speaker from Group / Metadata for URIs

Minor new function allows speakers to be un-grouped.
Tested with a 4 speaker network. No apparent problems with removing group masters or ungrouped speakers, the topology updates appear to correctly notify other controllers of the changes and they respond appropriately. There may be extra work to do to re-start playback on an orphaned group after removing master.

Added parameter to play_url to allow meta data to be set - this needed to allow the speakers to connect up to services (e.g. Tunein) to allow streams hosted by those services to play. Most useful in this context to play back the Favourite Radio stations discovered by get_favorite_radio_stations.

First commit ever using GIT so be gentle and I hope I have done it right.